1. Field of the Invention
The present invention relates to a satellite signal tracking system for Global Positioning System (GPS) receivers.
2. Description of the Prior Art
Positioning systems currently in service utilizing artificial satellites include a so-called Global Positioning System (GPS).
This positioning system, as the name implies, will cover the entire surface of the Globe by a total of 24 geodetic satellites when all of them are launched into six orbits at an altitude of approximately 20,200 km above the Earth, wherein four geodetic satellites are assigned for each orbit. Each geodetic satellite transmits a GPS signal containing navigational data for positioning to the Earth in a spread-spectrum system. The positioning is carried out with use of a GPS receiver arranged on the ground, on the sea as well as in the air by receiving GPS signals from a plurality of geodetic satellites, for example, by receiving GPS signals from three geodetic satellites for two-dimensional positioning and that from four geodetic satellites for three-dimensional positioning. In this way, based on the navigational data contained in the GPS signal from each geodetic satellite, position information of the receiving point such as a latitude, longitude and altitude at the time of reception can be reckoned on a real time base.
This GPS system was originally developed for U.S. military use, however a part of the GPS signal (C/A code) has been made available to the civil use. Therefore, it is possible to build navigation systems for motor vehicles, vessels, aircraft and the like by making use of the GPS signal.
A GPS receiver used for a vehicle-mounted navigation equipment starts a search operation to recapture the GPS satellite when the GPS satellite is screened from reception with a building and the like and the reception of the GPS signals is interrupted for more than a specified period of time (for example, one minute) in accordance with operational steps shown in a flowchart of FIG. 1.
The GPS satellites are transmitting GPS signals on a frequency of 1575.42 MHz, however, since they are not geostationary, the receiving frequency may shift due to the Doppler effect. The Doppler frequency shift caused by the orbiting GPS satellite is approximately .+-.5 KHz on the ground.
The Doppler effect may also be caused by the movement of the GPS receiver itself or, in case of a vehicle-mounted GPS receiver, by the movement of the vehicle. The Doppler frequency shift becomes the maximum of .+-.600 Hz when the vehicle moves at a speed of 60 m/sec. Further, a frequency shift of approximately .+-.3 KHz may occur at an oscillator of the GPS receiver. By adding these frequency shifts, the maximum frequency shift will become 5,000.+-.600.+-.3,000=.+-.8,600 Hz. This means that the satellite signal receiving frequency may shift to the maximum of .+-.8,600 Hz from the center frequency of 1575.42 MHz.
Accordingly, if an attempt is made to recover the lost GPS satellite by covering the maximum shift range of .+-.8,600 Hz. It is required for the GPS receiver to search for the satellite signal by swinging the search frequency widely within the maximum frequency shift range of .+-.8,600 Hz with the frequency of 1575.42 MHz in the center.
Generally, the GPS receiver adopts a phase-locked loop (PLL) circuit in its receiving circuit in order to synchronize the receiving frequency of the GPS receiver accurately with the transmitting frequency of the GPS satellite. There is no problem if a capture range of the PLL circuit is set widely enough to cover the maximum frequency shift range of .+-.8,600 Hz. However, in practice, due to structural limitations of the circuit configuration, the capture range of the PLL circuit is forced to be as narrow as .+-.150 Hz=300 Hz or so.
Therefore, in order to capture the satellite signals by covering the maximum frequency shift range of .+-.8,600 Hz in the receiving frequency, it is necessary for the GPS receiver to divide the search frequency of the PLL circuit into multiple steps and perform the search by switching the steps as shown in FIG. 2. It is assumed, for example, that one PLL capture range is .+-.150 Hz=300 Hz, a number of steps to be performed in searching will be 8,600/300=28.66=28. That is, the search frequency should be switched as many as 28 steps for one side of the transmission frequency of the GPS satellite, and the total of 56 steps are required to cover the overall search range for upper and lower sides.
As mentioned previously, since the satellite signal is transmitted in accordance with the spread-spectrum system, the received signal should be despread firstly to identify whether or not the signal received is the one transmitted from the target GPS satellite. It takes about one second for this identification, accordingly, 56 steps of searching operation require at least 56 seconds and the essential reckoning for the position cannot be performed within this period of time.
Furthermore, the GPS receiver needs to capture and track at least three, desirably four, GPS satellites for positioning. The most preferable configuration of the GPS receiver is to allot one receiving channel for each GPS satellite. However, in a commercial instrument such as the vehicle-mounted navigational equipment described above, it is common to employ a single receiving channel in terms of requirements such that accuracy in positioning, low production cost, miniaturization and the like, and thereby the single receiving channel is multiplexed by time-division to enable the GPS receiver to receive satellite signals from three or four GPS satellites in sequence. In case of recapturing, for example, N satellites for tracking by using a sequential receiving type GPS receiver, at least 56.times.N seconds are necessary for making one round of the recapturing operation as shown in FIG. 2.
If the signal from the GPS satellite is happened to be interrupted by a building and the like while, for example, the vehicle was stopped at a stop signal and that the PLL search frequency was locked right on the receiving frequency of the GPS satellite, it may take another 56.times.N seconds for the PLL search frequency to lock on the receiving frequency of the GPS satellite in the next. Accordingly, there has been a problem with the prior art GPS receiver as it takes a considerable time for recapturing a GPS satellite, thus resulting in excessive delay before restarting the positioning.
It is therefore an object of this invention to eliminate the problem encountered in the prior art system and to provide a satellite signal tracking system for use in a GPS receiver enabling it to recapture a GPS satellite within the shortest possible time.
It is another object of this invention to provide a satellite signal tracking system for use in a GPS receiver allowing it to perform a wide band search repeatedly by predicting the receiving frequencies of the GPS satellite at least from the second search and thereafter for enabling it to recapture the GPS satellite within the shortest possible time.